Patterning of wafers is a key process for producing a semiconductor device. As device geometries get smaller, processes such as dry plasma etching will see more usage. Also, with the introduction of new materials comes a requirement for more advanced and new etching gases and mixtures. The above need has been further underlined recently due to the increasing interest in 3-D integration. While two silicon surfaces may be aligned face-to-face, integration of more than 2 circuit levels requires Through-Silicon-Vias (TSV). The aspect-ratio of such connections is of order 50:1, compared to less than 2:1 for vias on a single silicon chip. This very high aspect ratio places increased demand on the performance of the etching process, of which a key parameter is the etching chemistry. This shrinking of device geometries also places more demand on the materials used in the construction of the device; requiring improved purity of the incoming materials so as to reduce the possibility of defects.
After creation of the transistors, they must be interconnected using a conductive metal. Historically, the metal wires were made of aluminum separated by SiO2. Recently, aluminum has been replaced by copper as the interconnect material and low dielectric materials replaced SiO2. This has resulted in increased performance by reducing the timing delay in the wiring.
Carbonyl sulfide (COS) has many uses, such as an intermediate product or as a fumigant, in the production of merceptants, urea, etc. COS is also considered important for plasma etching technology for semiconductor devices. However, in the presence of H2O, carbonyl sulfide (COS) and H2S are known corrosion promoters towards copper. The result is the formation of CuS, Cu2S, or CuO depending on the reacting species. Specifically, H2S has been found to be corrosive towards Cu both in the presence and absence of H2O. This has been seen both at high concentrations of H2S and at trace levels. In addition, it has been found that COS in the presence of H2O is also corrosive towards Cu with similar corrosion properties as that of H2S in the presence of H2O.
There are several known methods focusing on the synthesis of COS through different synthesis routes. However in all known methods, there are common impurities existing as a result of the COS synthesis depending on the type of the reactants being used in the process. The main impurities in COS are CO2, CS2 and H2S coming either from CS2 source itself or produced during the course of the process, for example by the undesired hydrolysis of COS with H2O. As discussed above, it is well known that the combination of H2O and H2S or COS cause corrosion to copper, which may be critical in electronics applications. It is rather difficult to remove H2S from COS since the two compounds have similarly volatilities, as shown in FIG. 1. Therefore, the major challenge in the COS purification process is to efficiently remove H2S, as well as other impurities, using specifically adsorbents for high removal efficiency.
Several studies have been performed to remove sulfur-containing compounds, such as carbonyl sulfide (COS), hydrogen sulfide (H2S), and CS2, from the feedstocks used to prepare synthesis gas (“syngas”) in an effort to prevent sulfur poisoning of the catalysts in the syngas preparation. Syngas is a gas mixture containing at least CO and H2 that may be obtained from coal or alkane feedstocks, such as CH4 or CH3CH3. Conventional methods to purify syngas feedstocks from these undesired impurities include chemical absorption using amines, metal sulfates, and metal oxides. See, e.g., Lee et al., Environ. Sci. Technol. 35 (11) (2001) 2352-2357 (disclosing that the most common means to remove acid gases such as H2S and CO2 is adsorption by aqueous solutions of various alkanolamines); Mandal et al., Chemical Engineering Science 61 (2006) 5440-5447 (adsorption of CO2 into 2-amino-2-methyl-1-proposal, MEA (monoethanolamine), and water); Maat et al., Separation Purification Technology 43 (2005) 183-197 (disclosing a method to selectively remove H2S over CO2 from gas streams using aqueous metal sulfate absorbent); Wang et al., Applied Surface Science 254 (2008) 5445-5451 (room temperature chemisorption of H2S on zinc oxide modified Al-substituted SBA-15); Miura et al., Ind. Eng. Chem. Res. 31 (1991) 415-419 (disclosing the use of iron oxide to simultaneously remove COS and H2S from coke oven gas); Wang et al., Water Air Soil Pollut 193 (2008) (disclosing H2S removal by mesoporous SBA-15 supported iron oxide).
These methods generally focus on removing H2S or the combination of H2S and COS to purify the feedstock for the syngas industry. Therefore, these methods generally do not clearly address how to efficiently remove H2S selectively over COS. Additionally, these methods do not address the purity levels required by the electronics industry.
Among these methods, metal oxides offer the clear advantages of efficiently removing H2S selectively over COS through metal oxide beds. See, e.g., U.S. Pat. No. 6,692,711 (disclosing that zinc oxide is more reactive towards H2S than COS). A main disadvantage with this method is that according to the reaction (for example), H2S+ZnOZnS+H2O, water is produced while sulfur is trapped in the zinc oxide bed. However, the H2O produced (depending on the amount of H2S being treated) may react with COS and form H2S again (for example), COS+H2OH2S+CO2.
A need remains to purify COS to levels suitable for use in the electronics industry.